The study and application of materials containing residues derived from different types of industries in public works and construction is one of the technological and research fields which have advanced the most in recent years. In fact there is an entire new generation of cementing materials for use in construction and public works based on recycling silica/alumina-rich residues such as silica fume and fly ash, among others. On the other hand, in recent years a clear tendency towards the development and application of construction materials involving a minimum energy cost and a minimum contaminant emission in their manufacture while at the same time being, as an added value, materials compatibles with the environment, or at best, materials which contribute to overcoming pressing problems such as the emission of greenhouse or toxic gases, has been observed. In this sense, lime (Ca(OH)2) is a very powerful cementing material since in addition to being compatible with the great majority of factory items (stone, brick, etc.), its hardening entails capturing atmospheric CO2.
Residues generated in the manufacture of acetylene from calcium carbide have been used from the beginning of the existence of these residues in the treatment of agriculture and gardening lands, in the bacterial inerting of sewage water and wastes and in the preparation of concretes and mortars for direct use in construction works or for the manufacture of pre-manufactured elements.
Some known inventions use these residues for the manufacture of different products applicable in the construction industry but none has the object of applying the residues in capturing gases or optimizing carbide lime characteristics to prevent the problems which the impurities of the latter cause, which has prevented the widespread use of this residue. Said optimization of carbide limes allows producing a cementing material specialized in the reduction and storage of greenhouse gases, an aspect not claimed in any known invention. Nor do inventions relating to carbide limes determine, like in this invention, the physical characterization of the residues, the form of preservation and transport and the manufacturing conditions and they therefore also do not even mention, claim or establish the means to achieve that objective.
There are also many studies and ample knowledge on the properties of the calcium hydroxide as a chemical compound, on slaked limes, hydraulic limes, and commercial air limes, but there are no in-depth studies of the residual sludges from the manufacture of acetylene from calcium carbide in terms of the characterization of its hydraulic and air reactivity and its carbonation kinetics aimed at using this residue as a construction material characterized by its CO2 absorbing environmental activity. Although there are some partial studies on physicochemical characteristics of carbide limes and on their pozzolanic activity or application as a cementing material, they always relate to non-optimized limes (without removing impurities). As indicated in this specification, there is no study on the effects of the process for optimizing lime proposed herein.
The documents found which could be pointed out as the relevant background for the discussion of the novelty of the invention are the following:
FR561352 (THOMMELIN, M.A-II), Aug. 3, 1923, which claims the use of residual limes from acetylene production in the manufacture of blocks for construction. They in no case propose the purification and optimization of carbide lime properties or its applications as a CO2 capturing material.
In the same manner, but by using carbide lime and other residual limes (for example, from the sugar-making industry) FR714380 (VERNEY, H-J-M.), Jul. 24, 1930, proposes the mixture with pozzolans to obtain a construction material with hydraulic properties. Similarly to the case discussed above, it does not propose, in any case, the purification and optimization of carbide lime paste or its exploitation as a cementing material for capturing greenhouse gases.
U.S. Pat. No. 1,635,212 (PREST-O-LITE Company, New Cork; Herrly, C. J.) Jul. 12, 1927, which claims the use of mixtures of carbide lime with cellulose and silicates for the manufacture of blocks which can be used in construction once calcined at temperatures of up to 700° C. It does not claim, in any case, the prior treatment and purification of carbide lime. The heating of the blocks to obtain the final product is not compatible with that described herein since in addition to the costs which it involves, it generates a significant volume of CO2 emission.
CH 237 590 A (DICKMANN MAX [CH]) 15 May 1945 (1945-05-15). It claims the use of residual sludges formed mainly by the calcium hydroxide (Ca(OH)2) which is generated in the chemical industries using calcium carbide (CaC2)+water (H2O) to manufacture acetylene (C2H2) gas in construction. It does not however propose any type of purification or optimization of the product prior to its use as a cementing material, nor does it describe any type of specific process for processing the carbide lime, storage, prevention of carbonation, or applications as a CO2 capturing element, use in preserving the artistic historical heritage, and it omits or ignores the irreversible aggregation properties of the residues used by the invention and does not assess the optimization of its hydraulic reactivity with respect to pozzolans. In summary, the use of residual limes is proposed simply for lowering costs and, as it does not attribute high cementing capacity to it, the use of other related materials from hydraulic cementing, such as hydraulic limes, cements and slag, is proposed to obtain quick setting and to increase the mechanical properties.
WO 99/18151 A (REBASE PRODUCTS INC [CA]; LILLEY MARTIN J [CA]; MEADE D MARK [CA]; MOR) 15 Apr. 1999 (1999-04-15), which claims the use of residues from calcium carbide as part of a compound based on any thermoplastic polymer which can further include any additive of those commonly used in the thermoplastic polymer transformation industry. It does not mention, in any case, the purification of these residues or their cementing function or the manufacture of mortars and concretes characterized by the mixture of cementing minerals, aggregates and water, and it also does not mention the use thereof in the construction industry.
Al-Khaja, W. A. (Engineering Journal of Qatar University, vol. 5, 1992, p. 57-67) and Al-Khaja, W. A. et al. (Resources Conservation and Recycling, 6, 1992, p. 179-190) studied the effects of the addition of carbide lime in the preparation of mortars (with and without Portland cement), observing that the mechanical performances thereof is slightly reduced when compared to mortars prepared with calcitic limes resulting from the calcination and hydration of limestones. However, the authors did not select or optimize or purify the carbide limes, as proposed in this invention, which, as has been discussed, could explain why the mortars with impure carbide limes have a worse mechanical behavior than those prepared with traditional calcitic limes.
On the other hand, the capacity of carbide limes (with impurities) for producing hydraulic cements once mixed with metakaolin and silica fume as described by Morsy, M. S (Ceramics-Silikáty, 49, 2005, p. 225-229) has been studied. However, the transformation rate of non-purified carbide lime is rather low, even after 28 days of reaction. The process of selecting, purifying and optimizing the carbide lime object of the present invention significantly improves the hydraulic reactivity of carbide lime as described in detail below.
Different methods of purifying carbide lime to thus enable appointing it for different industrial uses of high added value have also been proposed. These treatments described in EP1150919B1 (Of Pauw Gerlings, J & Hendrikus, M; CalciTech, 7 Nov. 2001), include: a) heating at 800° C., a high energy consumption and high economical cost method; b) filtration, a rather ineffective process in removing impurities which tends to present problems due to the small particle size of the carbide lime calcium hydroxide which blocks the filters; c) dissolving the carbide lime calcium hydroxide in water and separating the insoluble impurities. Such dissolution is performed in the absence (WO97/13723, Bunger et al., BUNGER AND ASSOCIATES, INC., 17 Apr. 1997; U.S. Pat. No. 5,846,500, Bunger et al., BUNGER AND ASSOCIATES, INC., 8 Dec. 1998; U.S. Pat. No. 5,997,833, Bunger et al., BUNGER AND ASSOCIATES, INC., 7 Dec. 1999) or in the presence of complexing agents and/or organic additives (EP1150919B1). In the first case, the volume of water needed is large given the low solubility of calcium hydroxide (its solubility product is 10−5.19; therefore its solubility at room temperature and pressure is about 2 g/l), which makes the use of this process in an economical manner difficult except in the cases in which the final product is of high added value (for example, in the manufacture of precipitated calcium carbonate for use in the paper industry). The addition of different types of complexing agents (for example, sorbitol or sucrose) increases the amount of Ca(OH)2 dissolved in water (up to 70 g/l), therefore the process is more efficient (EP1150919B1), but its drawbacks include, first, the costs of the additives used, and second, the presence of the latter which can interfere in the subsequent use of the purified carbide lime solution. On the other hand, dissolving calcium hydroxide crystals entails the loss of all the physical and microstructural characteristics of carbide lime, characteristics which are, as described below, essential for the different applications of said lime as a binding material with hydraulic capacity in the presence of alumino-silicates and highly reactive in capturing CO2 and other contaminating gases, this process producing an effect contrary to that sought in the present invention.
It therefore seems that the existing patents as well as the research work published up until now do not disclose or claim any of the aspects of the present invention.
Although the limes obtained both by traditional routes (limestone calcination and lime slaking) and by the latter route (carbide lime) are chemically formed by Ca(OH)2 virtually at >80%, their properties (reactivity with respect to CO2 or other gases such as SO2 and NOx, binding and hardening capacity, rheology, reactivity with respect to pozzolans or other compounds with silica and alumina, among others) are largely conditioned by a series of parameters such as:
a) Crystal size and its distribution
b) Crystalline morphology (habit)
c) Degree of agglomeration, as well as aggregate morphology and size
d) Specific surface area
e) Concentration of water in a paste
f) Content and type of impurities
considering all these aspects, dry limes and lime pastes (with an excess of water) from the acetylene industry where the different parameters characterizing them are quantified have been studied. Emphasis has been placed on comparing the characteristics of the selected carbide limes with those calcitic limes manufactured by means of calcination and slaking (hydration) of limestone has been emphasized since these are more common in industrial applications and in construction. In this sense, the carbide lime generally has significantly better physical and microstructural properties than those of conventional calcitic limes, especially due to their reduced particle size, planar morphology, low aggregation tendency and large surface area. These characteristics make carbide limes very reactive with respect to gas (CO2 and/or SO2) fixation and with respect to hydraulic processes (great capacity for solubilizing silica and alumina and precipitating calcium silicate and aluminate hydrates).
The study conducted for the development of this invention has revealed that the selected residue which is generated in the manufacture of acetylene from calcium carbide, commonly referred to as lime, in spite of having a chemical composition very similar to traditional calcitic hydrated limes, has however, in addition to relevant impurities, unique physical properties distinguishing them from all other limes, giving it a different physicochemical characterization.
Said physicochemical characteristics are summarized in Table 1 where they are compared with the characteristics of a calcitic hydrated lime, representative of those produced by calcination and hydration of limestones.
TABLE 1Physicochemical characteristics of the selected carbide lime and calcitic limes.LimeCarbide limeCalcitic limePercentage of solids 0.25 0.39(in the paste)Surface area37 m2/g11.1 m2/gPrimary particle size5 to 100 nm100 to 200 nmMean aggregate size7 μm9-15 μmPhasesPortlandite80%94%Calcite 6% 5%Others (calcium sulfite hydrate;14% 1% (alumino-alumino-silicate hydrates;silicates)inorganic and organic carbon)Composition (% by weight, except indication in ppm)SiO22.502 0.035A12O31.264 0.019Fe2O30.093 0.02MgO0.105 0.28CaO69.61476.006Na2O0.018 0K2O0.007 0TiO20.025 0S (ppm)623836Cl (ppm)223 0Ni (ppm)27 0Cu (ppm)3733Sr (ppm)15859Zr (ppm)17 0Loss on calcination24.723.6
The results of the different analyses and tests carried out allow indicating the following conclusions with respect to the characteristics of the selected carbide lime, the use of some additives and the cementing material obtained from the optimized carbide lime, also an object of the invention:                The selected carbide lime has morphology, habit, particle size and degree of aggregation characteristics giving it a very large surface area. Said physico-structural characteristics suggest that it is a material with a high gas (CO2 and SO2) capturing capacity and favors pozzolanic reactions.        The selected carbide lime has a portlandite (Ca(OH)2) crystal size typically less than 100 nm and generally in the range of 5-100 nm as the analyses by transmission electron microscopy demonstrate (FIG. 1a). The carbide lime is therefore a nanomaterial. The primary particle size is less than that of the calcitic air limes produced by calcination and hydration of limestone characterized between 100 and 200 nm.        The selected carbide limes have a low degree of aggregation, the aggregates formed being less than 10 μm in size, and generally between 2 and 8 μm (FIG. 1b). Furthermore, said aggregates are normally non-oriented particle aggregates and are therefore easily redispersible. In contrast, commercial calcitic air limes and particularly the more common lime powder typically include large aggregates (up to 20 μm) which tend to have oriented, therefore, irreversible aggregation. This gives them a relatively small surface area and, therefore, limited reactivity.        The small particle size and the low degree of aggregation thereof, as well as an eminently planar morphology of the hexagonal portlandite crystals in the selected carbide limes means that they have surface area values>30 m2/g, occasionally close to 40 m2/g, a value which doubles or even triples the surface area value of calcitic limes. Such a large surface area value indicates that the carbide lime will be extremely reactive.        The high reactivity thereof has, as an unwanted effect, an early carbonation during storage and transport if precautions are not taken (storage in perfectly hermetic containers).        The phases forming the selected carbide lime paste are: portlandite (≧80% by weight), with traces of calcite, carbonaceous particles (inorganic carbon, essentially graphite, and organic carbon (FIG. 1c), calcium sulfite hydrate (FIG. 1d) and calcium alumino-silicate hydrate. Furthermore, metals such as Ni, Cu, Sr and Zr are detected at concentrations which jointly exceed 200 ppm. The concentration of calcite increases during the drying of the lime paste (early carbonation) from values of 5 to 10% by weight to values of 20 to 25% by weight. Furthermore, said drying causes the oriented aggregation (irreversible) of the portlandite particles (which reduces their surface area). Therefore, in this invention, the use of dry carbide limes (powder) is rejected because the calcite in carbide limes would act as an inert material and because the oriented aggregation of portlandite crystals, result of said drying, reduces the reactivity.        Since carbide limes contain silica and alumina at concentrations ranging between 1 and 3% by weight, the presence of calcium alumino-silicate hydrate formed after solubilization of these compounds present in the selected carbide lime at high pH is detected. The existence of Ca in solution, result of dissolving the portlandite, finally brings about the precipitation of the calcium alumino-silicate hydrate. The presence of this alumino-silicate demonstrates that the selected carbide lime is slightly hydraulic, although it is not strictly a hydraulic lime.        The presence of impurities, especially organic carbon (in solution, in porous aggregates (FIG. 1c), and absorbed in the portlandite crystals), sulfides and heavy metals is a problem which has not been solved up until now and limits the industrial and technological use of such carbide limes which, on the other hand, have much better size, morphology and surface area characteristics than those of hydrated limes obtained by calcination and hydration of limestones.        
These characteristics can vary according to the purity and quality of the original residues and the processing used during the manufacture of the calcium carbide. Low quality raw materials and processing translate into a low performance carbide lime. Thus for example, Cardoso et al. (Powder Technology, 2009, vol. 195, p. 143-148) describe low surface area (11.3 m2/g) carbide limes having very reduced quality with an excess of graphite (5% by weight). The use of these low quality residues, although ruled out in this invention, is not recommended for the applications indicated herein. For this reason a study of the aforementioned parameters is essential to use and obtain a product having optimum performance by means of treatment according to the inventive method.
The invention relates to a method for treating carbide lime residues, ideally applicable to residues which preferably have the suitable initial properties in terms of relevant physicochemicals characteristics, such as the case of those selected in the preliminary work of this invention, so that the products manufactured therewith have optimum performance, such that after the treatment they are not potentially toxic or they are not able to release contaminating elements (heavy metals) and gases (hydrogen sulfide), they do not release soluble sulfates which can cause the destruction of construction materials by crystallizing in the porous interior thereof, and at the same time, such carbide limes optimized and purified by the method of this invention have a high cementing capacity by air-setting primarily, although the setting can be hydraulic in the presence of pozzolanic materials, and so that they can meet the function of capturing ambient CO2 and SO2 (which is described in the earlier patent E 09380047.2 by TRENZAMETAL, S.L.).
Firstly, the method consists of the fact that from the moment the selected residues are removed from the acetylene generator to the moment of their incorporation in the mixtures for manufacturing the final products, they permanently maintain the calcium hydroxide contained therein, decanted in the original aqueous solution without contact with the air or vacuum-packaged in hermetically closed containers if the original water content is less than 25% of the total weight and preferably if it is less than 35% of the final weight. Thus the negative effects due, on one hand, to the early carbonation, and on the other, to the reduction of the lime reactivity due to drying and the subsequent oriented aggregation of the calcium hydroxide particles, are prevented.
Furthermore the oxidation treatment of the residues with hydrogen peroxide or another oxidizing reagent (for example, oxygen or mixtures of oxygen and nitrogen gases, among others), or bubbling with air in which the CO2 has been previously removed, is incorporated. The process for producing air without CO2 according to this invention consists of isolating the calcium hydroxide saturated solution, making up the supernatant liquid in the previously decanted carbide lime residues, in containers and subsequently using one of the following processes: 1) passing the air taken from the environment, bubbling it through the saturated solution contained in the containers; 2) passing the ambient air through a series of vertical curtains of the saturated solution; and 3) passing the air inside a tunnel through a cloud of the micronized solution. Said process allows oxidizing the sulfides from the impurities of the carbon used in the manufacture of calcium carbide, susceptible of generating hydrogen sulfide (H2S) and sulfites until forming sulfates. The sulfur in a reducing medium (due to the presence of carbon residues giving the carbide lime a grayish hue) is hydrolyzed forming hydrogen sulfide and is subsequently oxidized forming sulfites (in an alkaline medium). The oxidation of sulfides and sulfites generates calcium sulfate which is subsequently treated according to the invention to form insoluble barium sulfate.
The oxidation treatment has significant effects on the cementing properties of the product since it also causes the oxidation of the organic carbon present in the carbide lime paste. Said organic matter inhibits the pozzolanic reactions which can take place upon mixing the carbide lime with different type of pozzolanic materials (metakaolin, slag, silica fume, fly ash, expanded glass, etc.) and other natural alumino-silicate materials such as clays. Such effect is achieved both by means of treatment with hydrogen peroxide or another oxidizing reagent (for example, oxygen or mixtures of oxygen and nitrogen gases, among others), and in a more economic and efficient manner by using air without CO2 obtained by means of the method described in this invention.
The method for optimizing and purifying carbide limes also contemplates the addition of barium hydroxide for the fixation of heavy metals (fundamentally Sr, Cd, Cu, Pb, Ni, and Zn) and transformation of calcium sulfates (result of the oxidation of the sulfur compounds according to the method described above) into insoluble barite which, upon precipitating, incorporates said contaminating metals.
In the case of adding barium hydroxide in excess, the latter would reinforce the air-setting capacity of the carbide lime by transforming into very insoluble barium carbonate with high cementing power.
The material obtained after the treatment and optimization of the carbide lime as described in the inventive step which is object of the invention, acquires its full cementing capacity mixed with common soil minerals, especially the more reactive and abundant ones such as kaolinite and smectites, obtaining compact aggregates having sufficient strength not only for compacting soils but also for forming pavements, floors and surface courses in streets, roads and highways.
The high reactivity of the material obtained after the treatment and optimization of the carbide lime makes it especially ideal as a material of high hydraulic cementing potential once mixed with pozzolans. The tests performed demonstrate that the hydraulic reactivity of the carbide lime optimized according to the inventive method is greater than that of non-optimized carbide lime and much greater than that of traditional calcitic limes prepared by means of calcination and slaking of limestone.
Construction elements, whether pre-manufactured or made in situ, and generally products manufactured with the material object of this invention, during their service life, once their hydraulic setting reaction has ended in the event that pozzolans are added or it is applied to alumino-silicate materials (soils), while the air-setting process continues, are a very powerful CO2 absorbent due to the continuous process for returning the calcium hydroxide (Ca(OH)2) to its original natural mineral composition of calcite (CaCO3) by means of the carbonation process according to the following reaction:Ca(OH)2+CO2→CaCO3+H2O
The carbide lime optimized according to the inventive method do not present the drawbacks observed in the past in relation to the low strength of the non-optimized carbide lime-based constructive elements (mortars), such as described in the literature (Al-Khaja et al. 1992, Resources Conservation and Recycling 6, 179-190).
Likewise, the carbide limes optimized according to the method proposed in the inventive step show a greater pozzolanic reactivity than the mixtures of pozzolan (for example, metakaolin and silica fume) and carbide lime with impurities (untreated) used in the past (Morsy, 2005, Ceramics-Silikáty, 49, 225-229). After 10 days of setting, a mixture of optimized carbide lime and metakaolin/expanded silica in a proportion by weight of 0.75 (with a solids/water ratio of 0.66) cured at room temperature in an atmosphere of 93% relative humidity caused the complete consumption of the lime calcium hydroxide and the mass precipitation of calcium silicate and aluminate hydrates. Similar mixtures with carbide limes not optimized according to the inventive method proposed herein did not allow obtaining the complete consumption of the portlandite even after 28 days of setting (Morsy, 2005, op. cit.).